1. Field of the Invention
The present invention relates to an automatic gain control circuit (hereinafter referred to as an AGC circuit) for use in a television receiver and a video tape recorder and, more particularly, to a peak AGC circuit.
2. Description of the Prior Art
As is well known in the art, an AGC circuit is used for always keeping a video detection signal outputted from a video intermediate frequency signal processing circuit at a constant level despite the variation of an input level, by automatically changing the gain of the video intermediate frequency signal processing circuit or a tuner. Various types of AGC circuits such as a mean AGC circuit, a peak AGC circuit and a keyed AGC circuit are known, as described in detail in the document" Circuit Design of Television Receiver, Rajio-Gijutsu-sha, Japan, 1968, pp. 181-187" and "Television and Image Technology Handbook, Ohm-sha, Japan, 1977, pp. 982-984". Of these types of AGC circuits, the peak AGC circuit is mainly used in recent years.
FIG. 1 is a circuit diagram showing an example of the conventional peak AGC circuit. Referring to FIG. 1, a video intermediate frequency signal processing circuit 11 includes a variable gain amplifier 12 and an amplitude demodulation circuit 13. A video intermediate frequency signal supplied to an input terminal 14 from a turner (not shown) is amplified by the variable gain amplifier 12, detected by the amplitude demodulation circuit 13 and then outputted from an output terminal 15 as a video detection signal. An AGC circuit 16 includes a differential amplifier 17 comprising transistors Q.sub.1 and Q.sub.2. The base of the transistor Q.sub.1 is supplied with the video detection signal outputted from the video intermediate frequency signal processing circuit 11 and the base of the transistor Q.sub.2 is supplied with a reference voltage V.sub.A.
When the voltage V.sub.1 (voltage of synchronous negative polarity in this example) of the video detection signal is greater than the reference voltage V.sub.A, the transistor Q.sub.1 is in a conduction state. As a result, a current mirror circuit consisting of transistors Q.sub.3 and Q.sub.6 is activated and a current mirror circuit consisting of transistors Q.sub.4 and Q.sub.5 and a current mirror circuit consisting of transistors Q.sub.7 and Q.sub.8 are inactivated. Assuming that the current of a constant current source 18 is I.sub.1, a current of I.sub.1 /50 is supplied to a capacitor C.sub.1 from the collector of the transistor Q.sub.6, in the case of R.sub.4 =50R.sub.1. Thus, the capacitor C.sub.1 is slowly charged so that the charge voltage V.sub.2 thereof is gradually increased.
On the other hand, when the voltage V.sub.1 of the video detection signal is smaller than the reference voltage V.sub.A, the transistor Q.sub.2 is in a conduction state and the transistor Q.sub.1 is in nonconduction of the transistors Q.sub.4 and Q.sub.5 and the current mirror circuit consisting of the transistors Q.sub.7 and Q.sub.8 are activated and the current mirror circuit consisting of the transistors Q.sub.3 and Q.sub.6 is inactivated. Assuming that R.sub.1 =R.sub.2 =R.sub.3 and R.sub.5 =R.sub.6, a current of I.sub.1 is supplied to the collector of the transistor Q.sub.8 from the capacitor C.sub.1. Thus, the capacitor C.sub.1 is rapidly discharged at a speed of fifty times as large as the case of increase.
FIG. 2A and FIG. 2B are diagrams showing changes of the voltage V.sub.1 of the video detection signal and the charge voltage V.sub.2 of the capacitor C.sub.1. Peaks of the video detection signal appear at points of a synchronous signal. The circuit shown in FIG. 1 is in a stationary state when a period of V.sub.1 &lt;V.sub.A, i.e., a period when the peak of the video detection signal crosses the reference voltage V.sub.A, is approximately one fiftieth of a period of V.sub.1 &gt;V.sub.A.
The charge voltage V.sub.2 of the capacitor C.sub.1 is received by the variable gain amplifier 12 as a control signal. The gain of the variable gain amplifier 12 is increased in response to the increase of the voltage V.sub.2 and decreased in response to the decrease of the voltage V.sub.2. Thus, the video detection signal outputted from the output terminal 15 is maintained at a constant level defined by the reference voltage V.sub.A, in spite of the level variation of the video intermediate frequency signal received at the input terminal 14. FIG. 3A to FIG. 3C are digrams showing such operation. If the level of the video intermediate frequency signal inputted suddenly becomes small, the level of the video detection signal outputted also becomes small in excess, so that the voltage V.sub.1 of a synchronous negative polarity of the video detection signal becomes much larger than the reference voltage V.sub.A. As a result, the charge voltage V.sub.2 of the capacitor C.sub.1 is slowly increased, to gradually increase the gain of the variable gain amplifier 12. In response to this, the peak of the voltage V.sub.1 of the video detection signal, i.e., the top of the synchronous signal, gradually approaches the reference voltage V.sub.A. When the top of the synchronous signal crosses the reference voltage V.sub.A a little bit (the stage shown in FIG. 2A), the stationary state is entered. If the level of the video intermediate frequency signal suddenly becomes large, the stationary state is entered through the operation opposite to the above. In this operation, the transition speed is approximately fifty times as fast as that of the above operation.
In the hereinbefore described conventional AGC circuit, increase of the capacitance of the capacitor C.sub.1 results in increase of stability to noise of the video detection signal since variation of the voltage V.sub.2 and therefore variation of the gain of the variable gain amplifier 12 are decreased. However, the response speed of the AGC circuit is decreased. On the other hand, if the capacitance of the capacitor C.sub.1 is decreased, the response speed of the AGC circuit is increased but the stability of a wave form of the video detection signal outputted is damaged. Many attempts are made in order to satisfy both the stability of the output waveform and the high speed of response by making the circuit structure of a filter circuit consisting the capacitor C.sub.1 further complicated. However, such attempts do not well succeed for the present.